The present invention relates generally to over-current protection circuits, which also are referred to herein as current limit circuits. More particularly, the present invention relates to an adjustable over-current protection circuit which achieves a fast response time and a high degree of accuracy despite large manufacturing process variations and despite large chip operating temperature variations, without use of a sense resistor or a separate external adjustment terminal.
Many electronic circuits include components which limit output current of the circuit to protect output transistors and/or other circuit components, such as load circuits driven by output transistors of the electronic circuits, from excessive output currents. Power amplifiers for driving low resistance loads usually include over-current protection circuitry to prevent the power amplifiers, especially output transistors therein, from being damaged by an overload current caused by a short-circuit of the amplifier output. Various techniques have been used to sense and limit output currents of various circuits and to protect output transistors from excessive output currents, i.e., from over-currents. These methods are used primarily in two classes of protection circuits: 1) those including a sense resistor to sense current in an output transistor, and 2) those which do not include such a sense resistor.
The first class of protection circuits, those with a sense resistor, conventionally place a small value resistor in the current path of the output transistor to sense the output current therein. For relatively large values of output transistor current, the voltage drop across the sense resistor reduces the available xe2x80x9cheadroomxe2x80x9d, i.e., the available voltage range or available voltage swing of signals such as the output signal of the electronic circuit. Also, the increased temperatures caused by power dissipation in the sense resistor can become excessive and cause damage to output transistors or other circuitry that is sensitive to high temperatures.
A prior art example of the first class of protection circuits is shown in FIG. 1, wherein protection circuit 10 senses the output current flowing through two sense resistors Rsc1 and Rsc2 by measuring the voltage across each of them. When the voltage across either Rsc1 or Rsc2 exceeds the base-to-emitter voltage (Vbe) of transistor Q3 or Q4, the current through transistor Q1 or transistor Q2, respectively, is limited and the output current therefore is also limited. For example, when the current through Rsc1 exceeds the base-to-emitter voltage of transistor Q3, then transistor Q3 xe2x80x9crobsxe2x80x9d base current from transistor Q1. This limits the current through resistor Rsc1 and hence through transistor Q1, and thereby protects output transistor Q1 and also limits the output current. The current through output transistor Q2 is limited in a similar way. The current limit value is determined by the values of Rsc1 and Rsc2, and because it Rsc1 and Rsc2 are fixed resistors, the current limit of the protection circuit must also be fixed, or there must be external terminals through which Rsc1 and Rsc2 may be adjusted. The current flowing through Rsc1 and Rsc2 causes those resistors to dissipate power and increase temperatures of nearby components on the integrated circuit chip. The tolerance of the current limit is no more accurate than the Vbe (base-to-emitter voltage) voltage of Q3 or the Vbe voltage of Q4. Because of the large tolerances, and because the Vbe of each of the transistors changes with temperature, the output current protection (or current limit value) is undesirably imprecise.
U.S. Pat. No. 5,739,712 by Fujii (April, 1998) discloses a number of other similar over-current protection schemes.
Protection circuits in the second class do not directly limit the current, but instead usually rely on a feedback circuit to control the current limit. An example of the second class of protection circuits is disclosed in U.S. Pat. No. 5,519,310 to Bartlett (May 1996), titled xe2x80x9cVoltage-to-Current Converter Without Series Resistor.xe2x80x9d Referring to FIG. 2 herein, which is a reproduction of FIG. 3 of the Bartlett patent, the output current lout of voltage-to-current converter circuit 12 is controlled by adjusting the current flowing through transistor M1. The differential amplifier OA2 compares the voltage across transistor M1 to a voltage between transistors M2 and M5, and adjusts the gate voltage of transistor M5 accordingly. The current flowing through transistors M3 and M5 is mirrored through transistor M4 and resistor R1. This mirrored current creates a feedback voltage VF voltage across resistor R1. The feedback voltage VF is compared with an input voltage Vin by differential amplifier OA1, which adjusts the gate voltage of M1 so as to increase Iout enough to force the voltage across resistor R1 to be equal to Vin. This causes Iout to be proportional to the current through resistor R1 and hence to Vin. However, the feedback delay in limiting the output current Iout may allow it to exceed the current desired to be established by Vin, and thereby cause chip overheating and damage to the output transistor M1 and other circuit components. The feedback delay also slows the overall response of a system including the circuit of FIG. 2.
Thus, there is an unmet need for an improved over-current protection circuit that does not dissipate excessive power and raise chip temperature, does not reduce operating voltage xe2x80x9chead roomxe2x80x9d, and does not cause substantial signal propagation delay.
It is an object of the present invention to provide an over-current protection circuit which achieves a higher degree of accuracy than previous protection circuits.
It is another object of the present invention to provide an over-current protection technique which more reliably protects output transistors of integrated circuits than previous techniques.
It is another object of the present invention to provide an over-current protection circuit or current limit protection circuit having faster response times than the closest prior art.
It is another object of the invention to provide an over-current protection circuit or a current limit circuit for protecting output transistors of integrated circuits which is adjustable over a wide range.
It is another object of the invention to provide an over-current protection circuit or a current limit protection circuit with high accuracy over large chip temperature variations and large manufacturing process variations.
It is also an object of the invention to provide a method of current control which can have symmetrical positive and negative current limits with a high degree of accuracy.
It is another object of the present invention to provide an over-current protection circuit in amplifier circuitry or output driver circuitry which drives a load circuit so as to provide a wide range of adjustability of output limit currents.
It is another object of the present invention to provide an over-current protection circuit which drives a load circuit so as to provide xe2x80x9con the flyxe2x80x9d adjustability of output limit currents supplied to the wide range of load circuits.
Briefly described, and in accordance with one embodiment, the present invention provides a method and apparatus for directly limiting the output current of an over-current protection circuit in an electronic device without being subject to the delays caused by feedback loops. By directly limiting the voltage at the gate of the output transistor of the over-current protection circuit to a maximum voltage value, the current through the output transistor is correspondingly limited. By generating the maximum voltage value in reference to a current which is representative of the maximum current desired for the over-current protection circuit, the desired over-current protection is accurately achieved.
In one embodiment of the invention, an over-current protection circuit is provided having an output transistor, the gate of which is driven by an input voltage that controls the current flowing through the output transistor. When the input voltage goes beyond a voltage limit, a voltage clamp circuit maintains the gate voltage of the output transistor at the voltage limit until the input voltage is no longer beyond the voltage limit. The voltage limit is generated in response to a current which is representative of the desired maximum current through the output transistor. According to another embodiment of the invention, the voltage clamp includes an amplifier circuit to increase the accuracy of a clamp which limits the gate voltage driving the output transistor so as to provide a xe2x80x9chardxe2x80x9d, rather than xe2x80x9csoftxe2x80x9d limiting of the output current of the amplifier circuit.